Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common life-threatening genetic diseases, and is a leading cause of renal failure. The majority of cases are caused by mutations in the PKD1 gene, which encodes for polycystin-1 (PC1). Recent evidence suggests that PC1 acts as a mechanosensor, receiving signals from the primary cilia, neighboring cells and extracellular matrix and transduces them into cellular responses that regulate proliferation, adhesion and differentiation that are essential for the control of renal tubules and kidney morphogenesis. PC1 is a large membrane protein that has an unusually long extracellular region (approximately 3000 aa) with a multi-modular structure. Proteins with a similar architecture have structural and mechanical roles. Our hypothesis is that PC1 is a mechano-transducer with a novel molecular architecture and elastic properties well-suited for sensing and transmitting distinct mechanical signals with a wide range of strengths. Consistent with this hypothesis we have found that most of the PC1 extracellular region is made of mechanically-elastic domains, providing direct support to the idea that the ectodomain may function as an effective force transmitter. Mutations may alter PC1's mechanical properties and so lead to the altered signal transduction and abnormal tissue development characteristic of ADPKD. To begin to understand the sensing mechanism of PC1, we will use using a combination of recombinant DNA and atomic force microscopy (AFM) techniques to assess the mechanical and biophysical properties of the extracellular region of normal and mutant forms of PC1. This technique offers the most direct way of studying the stability and elasticity of proteins that are exposed to mechanical forces and hence more closely approximate the conditions found in vivo. In Aim 1 we will use single molecule force spectroscopy to determine the mechanical properties of PC1-ectodomain. In Aim 2 we will determine whether missense mutations affect the mechanical and thermodynamic stability of PC1-ectodomain. In Aim 3 we will use protein engineering and computer simulations to develop a molecular model for PC1-ectodomain to examine the significance of its modular architecture and to enable a structural interpretation of the effects of the mutations. Our long term plan is to elucidate the structure and biophysical properties of the PC1 molecule and help us understand the effects of mutations on the PKD1 gene. These experiments should provide a solid basis to investigate the molecular mechanisms of the signaling events that link PC1 mutations with the ADPKD phenotype.